Electrochemical C-N coupling with perovskite hybrids toward efficient urea synthesis

Turning Waste into Wealth: Sustainable Production of High-Value-Added Chemicals from Catalytic Coupling of Carbon Dioxide and Nitrogenous Small Molecules

Carbon neutrality is one of the central topics of not only the scientific community but also the majority of human society. The development of highly efficient carbon dioxide (CO₂) capture and utilization (CCU) techniques is expected to stimulate routes and concepts to go beyond fossil fuels and provide more economic benefits for a carbon-neutral economy. While various single-carbon (C₁) and multi-carbon (C₂₊) products have been selectively produced to date, the scope of CCU can be further expanded to more valuable chemicals beyond simple carbon species by integration of nitrogenous reactants into CO₂ reduction. In this Review, research progress toward sustainable production of high-value-added chemicals (urea, methylamine, ethylamine, formamide, acetamide, and glycine) from catalytic coupling of CO₂ and nitrogenous small molecules (NH₃, N₂, NO₃^-, and NO₂^-) is highlighted. C-N bond formation is a key mechanistic step in N-integrated CO₂ reduction, so we focus on the possible pathways of C-N coupling starting from the CO₂ reduction and nitrogenous small molecules reduction processes as well as the catalytic attributes that enable the C-N coupling. We also propose research directions and prospects in the field, aiming to inspire future investigations and achieve comprehensive improvement of the performance and product scope of C-N coupling systems.

Emerging p-Block-Element-Based Electrocatalysts for Sustainable Nitrogen Conversion

Artificial nitrogen conversion reactions, such as the production of ammonia via dinitrogen or nitrate reduction and the synthesis of organonitrogen compounds via C-N coupling, play a pivotal role in the modern life. As alternatives to the traditional industrial processes that are energy- and carbon-emission-intensive, electrocatalytic nitrogen conversion reactions under mild conditions have attracted significant research interests. However, the electrosynthesis process still suffers from low product yield and Faradaic efficiency, which highlight the importance of developing efficient catalysts. In contrast to the transition-metal-based catalysts that have been widely studied, the p-block-element-based catalysts have recently shown promising performance because of their intriguing physiochemical properties and intrinsically poor hydrogen adsorption ability. In this Perspective, we summarize the latest breakthroughs in the development of p-block-element-based electrocatalysts toward nitrogen conversion applications, including ammonia electrosynthesis from N₂ reduction and nitrate reduction and urea electrosynthesis using nitrogen-containing feedstocks and carbon dioxide. The catalyst design strategies and the underlying reaction mechanisms are discussed. Finally, major challenges and opportunities in future research directions are also proposed.

A Reliable and Precise Protocol for Urea Quantification in Photo/Electrocatalysis

To comply with the trend toward green and sustainable development of the fine chemical industry, multitudinous promising technologies (e.g., photocatalysis and electrocatalysis) are beginning to dabble in the green synthesis of fine chemicals, particularly urea synthesis. Whilst numerous advances are made in mechanistic understanding, the low yield reported so far also imposes more stringent requirements on the reliability and anti-interference of the detection method. Herein, the applicability of frequently used methods for urea quantification is methodically compared. In terms of the experimental results, a precise and methodical protocol for urea quantification or evaluation in photo/electrocatalysis is explored and established, with emphasis on screening quantitative methods under specific conditions and indispensable isotopic tracing experiments. The budding urea photo/electrosynthesis urgently demands a rigorous protocol, including the rapid isotopic identification and evaluation criteria, capable of promoting healthy development in the future.

Accelerating electrochemically catalyzed nitrogen reductions using metalloporphyrin-mediated metal-nitrogen-doped carbon (M-N-C) catalysts

Herein, a series of transition metal coordinated metalloporphyrin-mediated M-N-C catalysts with single and dual metal atoms were prepared and characterized. Interestingly, these M-N-C catalysts exhibit accelerated N₂ reduction behaviors through electrochemical catalysis. At the potential of E = -0.4V (vs. RHE), the optimum catalyst Fe_0.95Ni_0.05TPP@rGO-800 shows excellent catalytic activity, and the NH₃ yield is 22.5 μg mg_cat^-1 h^-1, which is much higher than that of its single metal counterparts alone, and the faradaic efficiency is as high as 50.7%, which is better than those of most reported catalysts. These results provide an opportunity to further explore the efficient electrochemical synthesis of NH₃ from M-N-C materials in the future.

A Defect Engineered Electrocatalyst that Promotes High-Efficiency Urea Synthesis under Ambient Conditions

Synthesizing urea from nitrate and carbon dioxide through an electrocatalysis approach under ambient conditions is extraordinarily sustainable. However, this approach still lacks electrocatalysts developed with high catalytic efficiencies, which is a key challenge. Here, we report the high-efficiency electrocatalytic synthesis of urea using indium oxyhydroxide with oxygen vacancy defects, which enables selective C-N coupling toward standout electrocatalytic urea synthesis activity. Analysis by operando synchrotron radiation-Fourier transform infrared spectroscopy showcases that *CO₂NH₂ protonation is the potential-determining step for the overall urea formation process. As such, defect engineering is employed to lower the energy barrier for the protonation of the *CO₂NH₂ intermediate to accelerate urea synthesis. Consequently, the defect-engineered catalyst delivers a high Faradaic efficiency of 51.0%. In conjunction with an in-depth study on the catalytic mechanism, this design strategy may facilitate the exploration of advanced catalysts for electrochemical urea synthesis and other sustainable applications.

1T-MoS2 Nanosheets Coupled with CoS2 Nanoparticles: Electronic Modulation for Efficient Electrochemical Nitrogen Fixation

Electrocatalytic nitrogen reduction reaction (eNRR), a substitute process for the conventional Haber-Bosch for NH₃ production, has drawn tremendous attention due to its merits in mild conditions, abundant reactant sources, low energy consumption, and environmental protection. However, electrocatalysts for eNRR are still subjected to low catalytic activity and selectivity. Herein, we constructed a CoS₂/1T-MoS₂ heterostructure with CoS₂ nanoparticles uniformly loaded on 1T-MoS₂ nanosheets and applied it as an eNRR electrocatalyst for the first time. Theoretical calculation suggests that electron transfer from CoS₂ to 1T-MoS₂ across their contact interface optimizes the local electronic structure of 1T-MoS₂, where the electron-depletion region near CoS₂ is in favor of accepting lone-pair electrons from N₂ to enable N₂ absorption, and the electron-accumulation region near 1T-MoS₂ is conductive to break inert N≡N triple bonds. Unlike pure 1T-MoS₂, the potential-determining step (PDS) demonstrates a significantly lower energy barrier. In addition, the weak interaction between CoS₂/1T-MoS₂ and hydrogen discourages competitive hydrogen evolution reaction. As a result, CoS₂/1T-MoS₂ exhibited noticeably improved eNRR activity and selectivity, with an NH₃ yield of 59.3 μg h^-1 mg^-1 and a high Faradaic efficiency of 26.6%.

Construction of C–N bonds from small-molecule precursors through heterogeneous electrocatalysis

Energy-intensive thermochemical processes within chemical manufacturing are a major contributor to global CO₂ emissions. With the increasing push for sustainability, the scientific community is striving to develop renewable energy-powered electrochemical technologies in lieu of CO₂-emitting fossil-fuel-driven methods. However, to fully electrify chemical manufacturing, it is imperative to expand the scope of electrosynthetic technologies, particularly through the innovation of reactions involving nitrogen-based reactants. This Review focuses on a rapidly emerging area, namely the formation of C-N bonds through heterogeneous electrocatalysis. The C-N bond motif is found in many fertilizers (such as urea) as well as commodity and fine chemicals (with functional groups such as amines and amides). The ability to generate C-N bonds from reactants such as CO₂, NO₃^- or N₂ would provide sustainable alternatives to the thermochemical routes used at present. We start by examining thermochemical, enzymatic and molecular catalytic systems for C-N bond formation, identifying how concepts from these can be translated to heterogeneous electrocatalysis. Next, we discuss successful heterogeneous electrocatalytic systems and highlight promising research directions. Finally, we discuss the remaining questions and knowledge gaps and thus set the trajectory for future advances in heterogeneous electrocatalytic formation of C-N bonds.

Establishing the Principal Descriptor for Electrochemical Urea Production via the Dispersed Dual-Metals Anchored on the N-Decorated Graphene

Urea electrosynthesis under mild conditions starting from the adsorption of inert N₂ molecules has brought out a promising alternative experimentally to conquer its huge energy consumption in industrial Haber-Bosch process. The most crucial and inevitable reaction is the formation of urea precursor *NCON from *N₂ and CO based on the pre-selected reaction pathway, together with the following protonated processes. It is significant to comprehend their intrinsic intercorrelation and explore the principal descriptor from massive reaction data. Hereby, the authors study the dispersed dual-metals (homonuclear MN₃ -MN₃ moiety and heteronuclear MN₃ -M'N₃ moiety) anchored on N-doped graphene as electrocatalysts to synthesize urea. Based on the screened out 72 stable systems by ab initio molecular dynamics (AIMD) simulations as the database, six significant linear correlations between the computed Gibbs free energy and other important factors are achieved. Most encouragingly, the principal descriptor (ΔE(*NCONH)) is established because 72% low-performance systems can be filtered out and its effective range (-1.0 eV < ΔEE(*NCONH) < 0.5 eV) is identified by eight optimal systems. This study not only suggests that dispersed dual-metals via MN₃ moiety can serve as promising active sites for urea production, but also identifies the principal descriptor and its effective range in high-throughput methods.

Non-metal boron atoms on a CuB12 monolayer as efficient catalytic sites for urea production

Non-metal B atoms at the midpoint of the edges of the squares is confirmed to be the excellent catalytic sites on CuB12 monolayer presents superior catalytic activity thermodynamically and kinetically than the reported urea catalysts.

Dual-Silicon-Doped Graphitic Carbon Nitride Sheet: An Efficient Metal-Free Electrocatalyst for Urea Synthesis

Searching for an alternative nonhazardous catalyst for direct urea synthesis that avoids the traditional route of NH₃ synthesis followed by CO₂ addition is a challenging field of research nowadays. Based on first-principles calculations, we herein propose a novel electrocatalyst comprising of totally nonmetal earth abundant elements (dual-Si doped g-C₆N₆ sheet) which is capable of activating N₂ and making it susceptible toward direct insertion of CO into the N-N bond, producing *NCON* which is the precursor for urea production by direct coupling of N₂ and CO₂ followed by multiple proton coupled electron transfer processes. Remarkably, the calculated onset potential for urea production is much less than that of NH₃ synthesis and hydrogen evolution reactions, and also the faradaic efficiency is nearly 100% for production urea over ammonia, which promotes exclusive electrocatalytic urea synthesis by suppressing the NH₃ synthesis as well as hydrogen evolution reactions.